1. Field of the Invention
The present invention relates to a radio communications system and, more particularly, to a method and device for demodulating control signals in a radio communications system in which reference signals and control signals are transmitted according to code division multiplexing (CDM).
2. Description of the Related Art
The 3rd Generation Partnership Project (3GPP), which is a collaboration among organizations aiming to standardize radio communications systems, has been studying Long Term Evolution (LTE), which provides a high-speed, low-latency, and packet-optimized radio access technology, as a successor to current W-CDMA systems. In LTE, single-carrier transmission is adopted as the uplink access scheme in broadband radio access. With low PAPR (peak to average power ratio), the single-carrier transmission is excellent in power efficiency, compared with multi-carrier transmission such as Orthogonal Frequency Division Multiplexing (OFDM). Hence, the single-carrier transmission is an access scheme suitable for an uplink from a mobile station to a base station. A mobile station is also referred to as “user equipment” or “UE” such as a mobile terminal having limited battery capacity. A base station is also referred to as “Node B” or “eNB”.
Moreover, for uplink reference signal (also referred to as “pilot signal”) sequences, Constant Amplitude Zero Auto-Correlation (CAZAC) sequences are used (see 3GPP TS36.211 v1.2.1). The CAZAC sequences are sequences having constant amplitude in the time domain as well as in the frequency domain and also having zero autocorrelation except when the phase difference is zero (e.g. B. M. Popovic, “Generalized Chirp-Like Polyphase Sequences with Optimum Correlation Properties,” IEEE Transactions on Information Theory, Vol. 38, No. 4, pp 1406-1409, July 1992). Because of the constant amplitude in the time domain, the CAZAC sequences can achieve low PAPR, and because of the constant amplitude in the frequency domain, the CAZAC sequences are suitable for frequency-domain channel estimation.
When a CAZAC sequence is used for uplink reference signal sequences, Code Division Multiplexing (CDM) is used to multiplex the reference signals of multiple mobile stations (see 3GPP R1-060925, Texas Instruments, “Comparison of Proposed Uplink Pilot Structures For SC-OFDMA,” March 2006). In CDM of reference signals, users can use CAZAC sequences of the same length respectively, and orthogonality between the reference signals can be accomplished by a cyclic shift unique to each user (mobile station) or each antenna. Hereinafter, the cyclic shift will be described briefly.
FIG. 1 is a schematic diagram to describe cyclic shifts based on a CAZAC sequence. Referring to FIG. 1, assuming that a CAZAC sequence C1 is a sequence 1, a sequence 2 is made by shifting the sequence 1 rightward (in the drawing) and relocating the shifted-out part at the end of the sequence 1 to the top of the sequence 1. Moreover, a sequence 3 is made by shifting the sequence 2 rightward (in the drawing) and relocating the shifted-out part at the end of the sequence 2 to the top of the sequence 2. By sequentially shifting the sequence in a ring manner as described above, sequences 4, 5 and 6 are made. This is called cyclic shift, and CAZAC sequences generated by cyclic shifts are referred to as cyclic-shift sequences. Hereinafter, the cyclic-shift sequences will be represented by S1, S2 and so on by using numbers that indicate shifted amounts.
Since the autocorrelation value of a CAZAC sequence is always zero except when the phase difference is zero as mentioned above, orthogonality between multiple reference signals can be accomplished even in a multi-path environment by making the amount of a cyclic shift to be relocated from the end of a sequence to the top thereof equal to or larger than a supposed maximum delay path time. For example, in a propagation path model according to LTE, since the maximum delay path time is approximately 5 μsec and a single long block is 66.6 μsec long, it is possible to use, logically, 13 cyclic-shift sequences from the calculation of 66.6/5. However, it is thought that approximately six cyclic-shift sequences can be orthogonalized in actuality because an impulse response is broadened along a path due to the influence of a filter and the like (see 3GPP R1-071294, Qualcomm Europe, “Link Analysis and Multiplexing Capability for CQI Transmission,” March 2007).
In LTE, reference signals (hereinafter, abbreviated as “RS” where appropriate) for the uplink can be broadly classified into three types: data demodulation reference signal for demodulation of Physical Uplink Shared Channel (PUSCH), which mainly transmits data; control signal demodulation reference signals for demodulation of Physical Uplink Control Channel (PUCCH), which transmits a control signal; and reference signals for measurement of uplink channel quality, or reference signals for CQI measurement (hereinafter, referred to as “sounding RS” or “sounding reference signal”).
FIG. 2 is a format diagram showing an example of resource allocation in a slot including PUSCH and PUCCH, demodulation reference signals for PUSCH and PUCCH, and a sounding reference signal. One slot is composed of seven blocks. Resource blocks (RB) on the edges of the entire band are allocated to PUCCH. PUCCH and PUSCH are multiplexed by frequency division multiplexing (FDM). Additionally, one resource block includes 12 subcarriers.
Moreover, PUCCH and the demodulation reference signal for PUCCH, as well as PUSCH and the demodulation reference signal for PUSCH, are multiplexed by time division multiplexing (TDM) in their respective bands. The sounding reference signal is assigned a resource of the system bandwidth, independently of the demodulation reference signals for PUCCH and PUSCH.
In control signal (PUCCH) transmission as shown in FIG. 2, to obtain a larger frequency diversity effect, it is defined in standardization to use CDM, by which PUCCH users to be multiplexed are spread across the PUCCH bandwidth. In this event, orthogonality between the users can be accomplished as in the above-described CDM of reference signals, by using CAZAC sequences as spreading codes. Moreover, CDM is also used in multiplexing of users of the control signal (PUCCH) demodulation reference signals so that a certain number of CAZAC sequences can be secured without a reduction in the sequence length of the reference signals.
For channel estimation for multiple user equipments UE multiplexed by CDM, a frequency-domain cross-correlation method can be used (see, FIG. 2 in 3GPP R1-070359, NEC Group, “Definition of Cyclic Shift in Code Division Multiplexing,” January 2007). As an example, a description will be given of channel estimation for four user equipments UE1 to UE4.
FIG. 3 is a block diagram showing a basic configuration of a multi-user channel estimation device. Referring to FIG. 3, after a CP deletion section 20 deletes cyclic prefixes (CP) from received signals, a fast Fourier transform (FFT) section 21 transforms the signals into frequency-domain representations. Subsequently, a multiplication processing section 22 carries out complex multiplication of the frequency-domain received signals with a single CAZAC sequence which has been transformed into a frequency-domain representation similarly. An inverse fast Fourier transform (IFFT) section 23 retransforms the result of this multiplication into a time-domain representation, whereby a cross-correlation signal based on the respective cyclic shift delays assigned to the user equipments UE1 to UE4 can be obtained. In accordance with the uplink or downlink signal reception qualities thus estimated, data rate control is performed.
An example of a channel demodulation method used in a CDMA receiver is disclosed in JP2005-72927. In this receiver, path search is performed by using delay profiles that represent a distribution of signal power values corresponding to individual path delays. Moreover, this receiver includes channel estimation sections and channel demodulation sections correspondingly to a predetermined number of demodulation paths obtained by the path search. After phase adjustment of demodulated symbols is performed, the paths are combined by a RAKE section, thereby obtaining a combined demodulated symbol.
However, different multiplexing methods are used for uplink control signals and data signals, as in the above-described case where control signals are multiplexed by CDM and data signals are multiplexed by TDM/FDM. In such a case, if demodulation processing of control signals is performed by using a similar configuration that is used for demodulation processing of data signals, the processing cannot be performed suitably to the characteristics of the control signal transmission method, resulting in degraded efficiency.
Moreover, according to the configuration disclosed in JP2005-72927, channel estimation and channel demodulation need to be carried out for each user. Consequently, efficiency is reduced in throughput as well as in circuit scale.